This book traces the history of the ancient Khazar Empire, a major but almost forgotten power in Eastern Europe, which in A.D. 740 converted to Judaism. Khazaria, a conglomerate of Aryan Turkish tribes, was finally wiped out by the forces of Genghis Han, but evidence indicates that the Khazars themselves migrated to Poland and formed the craddle of Western (Ashkenazim) Jewry...
The Khazars' sway extended from the Black sea to the Caspian, from the Caucasus to the Volga, and they were instrumental in stopping the Muslim onslaught against Byzantium, the eastern jaw of the gigantic pincer movement that in the West swept across northern Africa and into Spain.
Thereafter the Khazars found themselves in a precarious position between the two major world powers: the Eastern Roman Empire in Byzantium and the triumphant followers of Mohammed. As Arthur Koestler points out, the Khazars were the Third World of their day, and they chose a surprising method of resisting both the Western pressure to become Christian and the Eastern to adopt Islam. Rejecting both, they converted to Judaism.
The second part of Mr. Koestler's book deals with the Khazar migration to Polish and Lithuanian territories, caused by the Mongol onslaught, and their impact on the racial composition and social heritage of modern Jewry. He produces a large body of meticulously detailed research in support of a theory that sounds all the more convincing for the restraint with which it is advanced.
Mr. Koestler concludes: "The evidence presented in the previous chapters adds up to a strong case in favour of those modern historians - whether Austrian, Israeli or Polish - who, independently from each other, have argued that the bulk of modern Jewry is not of Palestinian, but of Caucasian origin. The mainstream of Jewish migrations did not flow from the Mediterranean across France and Germany to the east and then back again. The stream moved in a consistently westerly direction, from the Caucasus through the Ukraine into Poland and thence into Central Europe. When that unprecedented mass settlement in Poland came into being, there were simply not enough Jews around in the west to account for it, while in the east a whole nation was on the move to new frontiers" ( page 179, page 180).
"The Jews of our times fall into two main divisions: Sephardim and Ashkenazim.
The Sephardim are descendants of the Jews who since antiquity had lived in Spain (in Hebrew Sepharad) until they were expelled at the end of the fifteenth century and settled in the countries bordering the Mediterranean, the Balkans, and to a lesser extent in Western Europe. They spoke a Spanish-Hebrew dialect, Ladino, and preserved their own traditions and religious rites. In the 1960s, the number of Sephardim was estimated at 500000.
The Ashkenazim, at the same period, numbered about eleven million. Thus, in common parlance, Jew is practically synonymous with Ashkenazi Jew." ( page 181).
In Mr. Koestler's own words, "The story of the Khazar Empire, as it slowly emerges from the past, begins to look like the most cruel hoax which history has ever perpetrated."

Mr. Koestler was an Ashkenazi Jew and took pride in his Khazar ancestry. He was also a very talented and successful writer who published over 25 novels and essays. His most successful book, Darkness at Noon, was translated in thirty-three languages.
As expected, The Thirteenth Tribe caused a stir when published in 1976, since it demolishes ancient racial and ethnic dogmas...At the height of the controversy in 1983, the lifeless bodies of Arthur Koestler and his wife were found in their London home. Despite significant inconsistencies, the police ruled their death a suicide...